Bash Loops and Conditionals

Bash, the default shell in many Linux distributions, offers powerful constructs for controlling program flow. This post focuses on two fundamental elements: if statements for conditional execution and for loops for iterative processing. Mastering these will significantly enhance your shell scripting abilities.

The if Statement

The if statement allows you to execute commands conditionally. Its basic structure involves a test condition followed by one or more commands that are executed if the condition evaluates to true. A simple example might look like this:

#!/bin/bash

number=10

if [ $number -gt 5 ]; then
    echo "The number is greater than 5"
fi

This script checks if the variable number is greater than 5. The -gt operator performs the numerical comparison. If true, the message is printed. The fi keyword marks the end of the if block. More complex conditions can be created using elif (else if) and else clauses.

The for Loop

for loops are ideal when you need to execute a block of code multiple times. Bash offers several ways to use for loops, including iterating over a sequence of numbers or a list of words. A common pattern iterates over a set of files:

#!/bin/bash

for file in *.txt; do
    echo "Processing file: $file"
done

This script processes all files ending in .txt in the current directory. Each filename is assigned to the variable file in each iteration, and the done keyword marks the end of the loop. Another way to use the for loop is by using an array:

#!/bin/bash

myArray=(
    "apple"
    "banana"
    "cherry"
)

for i in "${myArray[@]}"; do 
    echo "item is: $i"
done

Output:

item is: apple
item is: banana
item is: cherry

Further Reading

To delve deeper, explore Bash's case statement for multiple-condition decision-making and the while and until loops for conditions that are checked before and after each iteration, respectively. The Bash manual provides comprehensive documentation on these and other control flow structures.